1. Field of the Invention
The present invention relates to a radiation-sensitive resin composition and a pattern forming method.
2. Discussion of the Background
In the field of microfabrication represented by the manufacture of integrated circuit devices, lithographic technology enabling microfabrication with a line width of 0.20 μm or less has been demanded in recent years in order to increase the degree of integration. In a conventional lithographic process, near ultraviolet rays such as i-line radiation have been generally used. However, it is difficult to perform microfabrication with a line width of sub-quarter micron using near ultraviolet rays. Therefore, in order to enable microfabrication with a line width of 0.20 μm or less, utilization of radiation with a shorter wavelength has been studied.
As examples of such short wavelength radiation, a bright line spectrum of a mercury lamp, deep ultraviolet rays represented by excimer lasers, X rays, electron beams, and the like can be given. A KrF excimer laser (wavelength: 248 nm) or an ArF excimer laser (wavelength: 193 nm) are given particular attention.
As a resist applicable to such excimer laser radiation, a number of resists utilizing a chemical amplification effect between a component having an acid labile functional group and a component which generates an acid upon exposure to radiation (hereinafter referred to simply as “exposure”) have been proposed. The component which generates an acid upon exposure is hereinafter referred to as “acid generator” and the resist utilizing a chemical amplification effect is hereinafter referred to as “chemically-amplified resist”.
As a chemically-amplified resist, a resist containing a polymer having a t-butyl ester group of carboxylic acid or a t-butyl carbonate group of phenol and an acid generator has been proposed (see Japanese Patent Application Publication (KOKAI) No. 59-45439). In such a resist, the t-butoxycarbonyl group or t-butyl carbonate group in the polymer dissociates by the action of an acid generated upon exposure, whereby the polymer has an acidic group such as a carboxylic group or a phenolic hydroxyl group. As a result, exposed areas of the resist film become readily soluble in an alkaline developer.
Many general chemically-amplified resists contain a phenolic base resin. If such a resin is used, deep ultraviolet rays used as radiation for exposure are absorbed in the resin due to an aromatic ring contained therein and cannot sufficiently reach the lower layers of the resist film. Because of this, the amount of radiation is greater in the upper layers and is smaller in the lower layers of the resist film. This causes a resist pattern profile to have a trapezoid shape having a thin upper portion and a thick lower portion after development. No sufficient resolution can be obtained from such a resist film. Such a trapezoid resist pattern profile formed after development cannot give a desirable dimensional accuracy in the succeeding steps such as an etching step and an ion implantation step. In addition, if the resist pattern profile is not a rectangle in which the upper side and the sidewall make almost right angle, the resist disappears faster during dry etching, making it difficult to control etching conditions.
A resist pattern profile can be improved by increasing the radiation transmittance through the resist film. A (meth)acrylate resin represented by polymethylmethacrylate is a highly desirable resin from the viewpoint of radiation transmittance, because the (meth)acrylate resin has high transparency to deep ultraviolet rays. For example, a chemically-amplified resist using a methacrylate resin has been proposed (see Japanese Patent Application Publication (KOKAI) No. 4-226461).
However, this composition has insufficient dry etching resistance due to the absence of an aromatic ring, although the composition excels in microfabrication performance. It is difficult to perform etching with high accuracy using this resin composition. Thus, the composition cannot be regarded as having both transparency to radiation and dry etching resistance.
As a means to improve dry etching resistance of the chemically-amplified resist without impairing transparency to radiation, a method of introducing an aliphatic ring into a resin component in the resist instead of an aromatic ring is known. For example, a chemically-amplified resist using a (meth)acrylate resin having an aliphatic ring has been proposed (see Japanese Patent Application Publication (KOKAI) No. 7-234511).
As acid labile functional groups, this resist includes a resin having a group which is comparatively easily dissociated with a general acid (e.g. an acetal functional group such as a tetrahydropyranyl group) and a group which is comparatively difficult to be dissociated with an acid (e.g. a t-butyl functional group such as a t-butyl ester group and t-butylcarbonate group). However, the resin component having the former functional group which is comparatively easily dissociated with a general acid has a problem of poor storage stability, although the basic resist properties such as sensitivity and pattern profile are excellent. On the other hand, the resin component having the latter functional group which is dissociated with an acid only with difficulty has excellent storage stability, but its basic resist properties, particularly sensitivity and pattern profile, are poor. Moreover, inclusion of an aliphatic ring in the resin component of this resist results in poor adhesion to substrates due to an extreme increase in hydrophobicity of the resin.
When forming a resist pattern by using a chemically-amplified resist, the resist is usually treated with heat after exposure in order to promote dissociation of the acid labile functional group. In such a case, fluctuation of the resist pattern line width due to fluctuation of the heating temperature is unavoidable. However, in order to respond to miniaturization of the integrated circuit devices in recent years, development of a resist having only small temperature dependency, that is, a resist which shows only small fluctuation in the line width due to the fluctuation of the heating temperature after exposure, has been demanded.
In addition, an acid generator is known to significantly affect the functions of a chemically-amplified resist. Presently, onium salt compounds which generate an acid at a high quantum yield and exhibit high sensitivity upon exposure are widely used as an acid generator for chemically-amplified resists. As such onium salt compounds, triphenylsulfonium hexafluoroantimonate, triphenylsulfonium naphthalenesulfonate, cyclohexylmethyl(2-oxocyclohexyl)sulfonium trifluoromethanesulfonate, and the like are used.
Most of these conventional onium salt compounds, however, do not exhibit satisfactory sensitivity. Although some compounds may exhibit comparatively high sensitivity, they are not necessarily satisfactory in overall resist performance such as resolution, pattern profile, and the like.